Today’s cars have become like a mobile network of control modules.  These control modules operate everything from the powertrain, braking, steering and suspension system to climate control, lighting, entertainment, communications and navigation. The technology is mostly incomprehensible to the average motorist, yet it provides all kinds of functions and capabilities that were not even on the radar a decade ago: things like Bluetooth connectivity, hands-free communication and email, automatic emergency braking, blind spot detection, adaptive cruise control, stability control, electronic steering and even key-less smart fobs that allow the vehicle to sense your approach, automatically unlock the doors for you and wake up the onboard electronics so you can be on your way.

To make such wonders possible, automotive engineers have created specialized control modules for all kinds of applications. Many motorists are somewhat familiar with the main modules in a vehicle such as the Powertrain Control Module or PCM, which used to be referred to as the Engine Control Module (ECM) or Engine Control Unit (ECU) because it was the computer that controlled engine functions such as spark timing, fuel mixture and emissions. PCM serves as a more descriptive term because the PCM on many vehicles also controls the transmission, which is part of the powertrain.

A Transmission Control Module (TCM) would be a separate control module for the transmission. It interacts with the PCM or ECM to make sure the transmission shifts at the appropriate speed and load.

The Body Control Module (BCM) is yet another major module that usually handles multiple tasks ranging from lighting and other electrical accessories to climate control, keyless entry, anti-theft duties and managing communications between other modules. The functions can vary greatly depending on the year, make and model of vehicle, and even its list of options.

This brings us to the “other” modules.  These modules aren’t very well known and most people don’t know about them until one fails and they have to get it replaced. These modules have all kinds of strange and confusing acronyms as each carmaker has come up with its own unique list of acronyms for the various modules they use in their vehicles.

A Real Gem
A typical example is a Ford GEM module (Generic Electronic Module). Ford started using these in the mid-to-late 1990s in various cars, minivans and light trucks. It is essentially a body control module it terms of what it actually does, though the list of control functions will vary depending on the vehicle application and its options. Some of the control functions include interior lighting, daytime running lamps, power windows, warning chimes and lamps, rear window defroster, windshield wipers and washers, perimeter anti-theft alarm, remote keyless entry and battery saver functions. The module may be located behind the fuse panel under the dash (Ford F-series trucks) or in the engine compartment near the power center.

If a Ford GEM module has to be replaced (which can happen if water infiltrates and corrodes the electronics inside the black box), it usually requires a part number that is specific to the customer’s vehicle. To further complicate matters, Ford says their GEM modules need to be programmed after they have been installed so they will function correctly. But some are simply plug-n’-play and will work right out of the box.
Many modules on many different makes and models of vehicles do require either reprogramming for a specific vehicle application or VIN code, or have to undergo some type of initialization or learning procedure (which may require a scan tool) after they have been installed before they will function normally. If a DIY customer doesn’t know this, they may think the replacement module you sold them is no good and bring it back with a warranty claim.

Play It Again SamAnother example of modularization would be the Mercedes “SAM” modules that divide up many of the subsystem electrical control functions in Mercedes C-Class and E-Class cars. There are two of these “System Acquisition/Activation Modules” in each car, a rear module (SAM-R) located in the trunk for electrical functions in the rear portion of the vehicle such as taillights, rear window defroster, door locks, etc.) and a front driver side module (SAM-D) in the engine compartment for the headlamps, front turn signals, wipers and other accessories. What each module controls will vary depending on the model year car and how it is equipped. The Mercedes SAM modules seem to be rather troublesome and can be easily damaged by voltage overloads and even battery disconnects. They also have to be reprogrammed after they have been installed.

Module ManiaListing all of the vehicle specific submodules would take too long. Instead here’s a short list of “other” modules classified by what they do. Many of these modules have a single dedicated function to perform, so they are relatively simple. But others can be nearly as complex as a PCM. To make matters worse, most of these modules may be located virtually anywhere inside the vehicle. Space is tight inside today’s electronics-packed vehicles, so engineers are often forced to locate the module wherever they can find a spot that hasn’t already been taken by something else. Finding a module’s location often requires looking it up on an illustrated component guide or wiring diagram.

Some of these other modules include:
● ABS/traction control/stability control module
● Airbag (SRS) module
● Alarm module (or chime module) for anti-theft system
● Cruise control module (if not integrated within the PCM)
● Electronic steering module
● Fuel pump control module
● Injector driver module (such as FSD/PMD modules on GM diesel engines)
● Instrument cluster control module (which may be part of the cluster itself or a separate black box)
● Keyless entry module
● Lighting module
● Remote start/immobilizer module
● Suspension control module
● Transfer case module (4WD)
● Wiper motor control module
● Vehicle communication module (such as GM OnStar module)
● Plus all kinds of “mini” modules for power windows, power seats, heated/cooled seats, power sliding doors, door locks, sunroofs, air flow control doors inside the Heating Ventilation Air Conditioning (HVAC) system, and so on.

More Modules Less WiringMuch of the hard wiring in today’s vehicles also has changed as a result of modularization. Many conventional wiring circuits have been eliminated altogether and replaced by Controller Area Networks (CAN) that allow various modules to share data and interact in ways that previously required hard wired connections or were not even possible. CAN networks started to appear in domestic cars back in 2003, and on some imports as far back as 1992 (Mercedes). Since 2008, CAN has been standard on all cars and light trucks sold in the U.S.

The basic idea behind CAN is that it allows data from many different systems to be shared via a common communication link, the data bus. Actually, most vehicles have two or three of these data buses that operate at different speeds (baud rates). Some share data at high speed and others share less important information at lower speeds. The data is coded so each module knows what to read and what to ignore.
The concept sounds complicated because it is. But it also simplifies the wiring by reducing the number of individual hard wired circuits that are needed in an accessory laden vehicle. That saves weight, bulk and cost (copper is rather expensive these days). Today’s average car probably has more than a mile and a half of wiring, according to one Delphi engineer. Without CAN, it could be much, much more.

Smarter Than The People Who Use ItOne of the benefits of so many modules is that today’s cars are smarter than ever before. The modules manage not only the powertrain, steering, suspension, brakes, climate control system and other subsystems, but also communications, navigation and safety (which can require very high data rates).

Active safety systems such as Volvo’s “City Safety” automatic braking system (introduced back in 2010 on the Volvo XC60) has an infrared laser camera mounted at the top of the windshield to monitor the road ahead. The camera is also used for adaptive cruise control and lane departure warning. It looks for reflective surfaces such as the taillights of another vehicle to identify obstacles and calculate the distance to that vehicle. If the City Safety system determines the rate of closure may result in an accident, it flashes an audible and visual warning to the driver. It also preloads the brakes in anticipation the brakes will be applied. If the driver fails to react in time, the system takes over and automatically applies the brakes, stopping the vehicle before it hits the object in front of it.

Even something as simple as opening a sliding side door on a minivan has changed. Opening a side door on a minivan used to be a simple manual task. You grab the door handle, pull it and slide the door open. With power sliding doors, it’s an entirely different process. Pressing the door switch sends a request to the Body Control Module (BCM), which then forwards an activation command to the power door control module that unlatches the door and energizes a small electric motor to pull the door open — but only if the transmission is in Park and the vehicle is not moving (a safety feature that is smarter than many of the people who are using it!).

GM’s OnStar system can monitor vehicle performance and even perform remote diagnostics when a problem arises. OnStar can even shut down the vehicle remotely if a vehicle is stolen and pinpoint its location for the police using GPS (Global Positioning System).

When Modules Fail
This highly sophisticated and complicated technology is wonderful as long as it is working. But when something goes amiss such as a module failure, communications bus failure or sensor fault, it can cause all kinds of problems sometimes in seemingly unrelated systems. The failure of a steering angle sensor may affect not only the electronic steering but also the stability control system since both need to know the position and turn rate of the steering wheel.

Diagnostics has become a major challenge for today’s technicians, and is totally beyond the abilities of most DIYers. The reason why is that it takes sophisticated diagnostic equipment, the know-how to use that equipment and lots of experience to correctly diagnose many module-related faults. Many modules are replaced unnecessarily because the real problem was misdiagnosed (things like bad grounds, loose or corroded wiring connections or low voltage).

Like the PCM, most modules have some type of self-diagnostics that should detect and report failures. The fault will log a diagnostic trouble code (DTC) and that code will be stored somewhere (the PCM or BCM) so it can be retrieved later with a scan tool.

An inexpensive DIY scan tool that can read OBD powertrain codes (“P” codes) and CAN communication faults (“U” codes) usually cannot access body codes (“B” codes) and other subsystem codes (though a few can read ABS and air bag codes). What is usually required is either a factory scan tool (which is expensive and only works on one make of vehicle) or a professional level scan tool (which is expensive but is supposed to cover a broad range of makes and models). But as many technicians have discovered, even some professional scan tools can’t access everything. There are often gaps in the tool’s data base that prevent it from reading certain subsystems and codes, or prevent it from running certain system self-tests. In such cases, the only way to access the information is with a dedicated factory scan tool.

The availability of modules also is a major issue, especially for older vehicles that car dealers no longer stock parts for. Various aftermarket suppliers can often provide remanufactured modules for many applications, but there are also a lot of gaps in coverage — and some modules may not be available anywhere except a salvage yard.

There are aftermarket companies that specialize in repairing all kinds of electronic modules, and this may be a repair option if a replacement module for a specific application is unavailable from aftermarket sources or a car dealer. The only drawback here is that some modules may be so badly corroded or damaged that they can’t be repaired. There also is the time-delay of sending the module in to have it repaired, and waiting to get it back (plus the added cost of shipping it).

One final comment about modules is this: You can’t tell much about a module’s condition or its ability to function by its external appearance unless it shows obvious signs of corrosion or damage (such as flood or fire damage, or physical damage from an accident). If there is no code that indicates the module has failed, a bad module is typically diagnosed by a process of elimination. Everything else is ruled out first (such as bad grounds, wiring faults, low voltage, bad sensor inputs, etc.) until the only remaining cause is the module itself. Many DIYers (and even some pros) don’t want to take the time (or don’t know how) to do the proper diagnostics, so they assume the problem must be a bad module because the module is the most complicated component in the system or circuit. That explains why electronic module returns are so high, and why so many modules that are returned under warranty have no fault found when they are tested by the supplier or remanufacturer.